Geochemistry and Sr-Nd-Pb isotopic characteristics of the Mugouriwang Cenozoic volcanic rocks from Tibetan Plateau: Constraints on mantle source of the underplated basic magma

摘要

The Mugouriwang Cenozoic volcanic rocks exposed in the north Qiangtang Block of Tibetan Plateau are mainly composed of basalt and andesitic-basalt, both characterized by the lower SiO_2(51%—54%), high refractory elements (i.e. Mg, Cr, Ni) as well as the moderate enrichment in light rare earth elements (LREE) relative to a slight depleted in Eu and high strength field elements (HFSE, i.e. Nb, Ta, Ti). Besides, the fairly low Sm/Yb value (3.07—4.35) could signify that the rocks should be derived directly from partial melting of the spinel Iherzolite at the upper part of the asthenosphere. These rocks have radiogenic Sr and Pb (~(87)Sr/~(86)Sr = 0.705339 to 0.705667; ~(208)Pb/~(204)Pb = 38.8192 to 38.8937; ~(207)Pb/~(204)Pb = 15.6093 to 15.6245; ~(206)Pb/~(204)Pb = 18.6246 to 18.6383), and non-radiogenic Nd (~(143)Nd/~(144)Nd = 0.512604 to 0.512639; ε_(Nd) = +0.02 to -0.66) in agreement with those values of the BSE mantle reservoir. The DUPAL anomaly of the rocks can be evidently attested by the Δ8/4Pb = 66.82 to 74.53 , Δ7/4Pb = 9.88 to 11.42, ΔSr > 50, implying that the Mugouriwang volcanic rock is likely to be generated by partial melting of a Gondwana-bearing asthenospheric mantle ever matasomatised by the fluid from subduction zone. Depending on the previous study on the high-K calc-alkaline intermediate-felsic volcanics in the study area, this paper proposed that the fluids derived from the subducted Lhasa Block metasomatised the asthenosphere beneath the Qiangtang Block, and induced its partial melting, and then the melt underplated the thickened Qiangtang lithosphere and caused the generation of the Cenozoic adakite-like felsic magmas in the Qiangtang region.